Progressive cavity pumps, also called Mono pumps, PCP pumps, or Moineau pumps, are a type of displacement pumps which are commercially available in a number of designs for different applications. In particular, these pumps are popular for pumping high-viscosity media. Typically, such pumps include a usually metallic helical rotor (in what follows called the inner rotor) with Z number of parallel threads (in what follows called thread starts), Z being any positive integer. The rotor typically runs within a cylinder-shaped stator with a core of an elastic material, a cavity extending axially through it being formed with (Z+1) internal thread starts. The pitch ratio between the stator and rotor should then be (Z+1)/Z, the pitch being defined as the length between adjacent thread crests from the same thread start.
When the geometric design of the threads of the rotor and stator is in accordance with mathematical principles written down by the mathematician Rene Joseph Louis Moineau in, for example, U.S. Pat. No. 1,892,217, the rotor and stator together will form a number of fundamentally discrete hollows or cavities by there being, in any section perpendicular to the centre axis of the rotor screw, at least one point of full or approximately full contact between the inner rotor and the stator. The central axis of the rotor will be forced by the stator to have an eccentric position relative to the central axis of the stator. For the rotor to rotate about its own axis within the stator, also the eccentric position of the axis of the rotor will have to rotate about the centre axis of the stator at the same time but in the opposite direction and at a constant centre distance. Therefore, in pumps of this kind there is normally arranged an intermediate shaft with 2 universal joints between the rotor of the pump and the motor driving the pump.
The pumping effect is achieved by said rotational movements bringing the fundamentally discrete cavities between the inner surfaces of the stator and the outer surfaces of the rotor to move from the inlet side of the pump towards the outlet side of the pump during the conveyance of liquid, gas, granulates etc. Characteristically enough, internationally these pumps have therefore often been termed “PCPs” which stands for, in the English language, “Progressive Cavity Pumps”. This is established terminology also in the Norwegian oil industry, for example.
The volumetric efficiency of the pump is determined mainly by the extent to which these fundamentally discrete cavities have been formed in such a way that they actually seal against each other by the relevant rotational speed, pumping medium and differential pressure, or whether there is a certain back-flow because the inner walls of the stator yield elastically or because the stator and rotor are fabricated with a certain clearance between them. To increase the volumetric efficiency, progressive cavity pumps with elastic stators are often constructed with under-dimensioning in the cavity, so that there will be an elastic squeeze fit.
Not very well known and hardly used industrially to any wide extent—yet described already in said U.S. Pat. No. 1,892,217—are designs of progressive cavity pumps in which a part, like the one termed stator above, is brought to rotate about its own axis in the same direction as the internal rotor. In this case the part with (Z+1) internal thread starts may more correctly be termed an outer rotor. At the same time it will then be natural to use the term inner rotor about the part which corresponds to the more usual rotor with an external screw and Z thread starts. By a definite speed ratio between the outer rotor and the inner rotor, both the inner rotor and the outer rotor may be mounted in fixed rotary bearings, provided the rotary bearings for the inner rotor have the correct shaft distance or eccentricity measured relative to the central axis of the bearings of the outer rotor.
A limitation to the gaining of ground of such early-described solutions has probably been that an outer rotor needs to be equipped with dynamic seals and rotary bearings, which is avoided completely when a stator is used. It is also likely that the potential increase in rotational speed and consequent increase in capacity enabled by the fact that the mass centres of both rotors will lie near the rotary axis have been overlooked or underestimated. Besides, an intermediate shaft and universal joints may, in principle, be avoided when the stator is replaced with an outer rotor.
In U.S. Pat. No. 5,407,337 is disclosed a Moineau pump (here called a “helical gear fluid machine”), in which an outer rotor is fixedly supported in a pump casing, an external motor has a fixed axis extending through the external wall of the pump casing parallel with the axis of the outer rotor in a fixed eccentric position relative to it, and the shaft of the motor drives, through a flexible coupling, the inner rotor which has, beyond said coupling, no other support than the walls of the helical cavity of the outer rotor, the material assumedly being an elastomer. In this case the rotation of the outer rotor is driven exclusively by movements and forces at the contact surfaces of the inner cavity against the inner rotor. A drawback of this solution is that if there is considerable clearance at or elastic deflection of the contact surface, the inner rotor or the outer rotor will be moved more or less away from its ideal relative position. Further, by increasing load, the driving contact surface between the inner and outer rotors will be moved constantly closer to the motor and thereby force the inner rotor more and more out of parallelism relative to the axis of the outer rotor, so that over the length of the outer rotor, the inner rotor will contact the outer rotor on diametrically opposite sides with consequent friction loss, wear on rotors and motor coupling and also possible signs of wedging. Vibrations, erratic running and reduced efficiency may also be expected.
In U.S. Pat. No. 5,017,087 as well as WO99/22141 inventor John Leisman Sneddon has shown designs of Moineau pumps, in which the outer rotor of the pump is enclosed by and fixedly connected to the rotor of an electromotor whose stator windings are fixedly connected to the pump casing. In these designs the outer and inner rotors of the pump are both fixedly supported at both ends radially in the same pump casing, so that the outer and inner rotors of the pump function together as a mechanical gear, driving the inner rotor at the correct speed relative to the outer rotor which, in turn, is driven by said electromotor. In this case as well, signs of wedging between the inner and outer rotors may arise, in particular if solid, hard particles seek to wedge between the inner and outer rotors where these have their driving contact surfaces. Besides, a disadvantage of an inner rotor fixedly supported at both ends is that if the pumping medium is of a kind which must be separated from contact with the bearings, independent dynamic seals will be needed at both ends for both the inner rotor and the outer rotor, as these do not have a common rotary axis.
In U.S. Pat. No. 4,482,305 is shown a pump, flow gauge or similar according to the PCP principle with inner and outer rotors. Here is used a wheel gear outside the pump rotors which ensures a stably correct relative rotational speed between the inner and outer rotors, independently of internal contact surfaces between them. This ensures smoother running, in particular by great pressure differences and/or spacious clearances—which may be necessary to achieve a gradual pressure increase when compressible media are pumped. However, it is assumed here as well that there are dynamic seals and radial bearings at both ends of the inner rotor. The dynamic seal for the outer rotor is also complicated by the diameter of the sealing surface having to be large enough to allow an internal passage for both the pumping medium and the bearing shaft on the extension of the active helical part of the inner rotor.
In the Norwegian patent application No. 20074591 is indicated a method of stabilizing the flow rate and outlet pressure in a progressive cavity pump with internal and external rotors intended for pumping compressible media. According to this document, signs of sudden cyclical back-flows of pumping medium in consequence of compression during the adjustment to the outlet pressure can be effectively limited by letting the defined pump cavity which is, at any time, the closest to the outlet side be allowed to have a substantially larger continuous leakage flow than the other pump cavities. To be as effective as possible, this leakage flow must be planned and be built into the construction of the outer and/or inner rotor(s) in each individual case. The document does not indicate a way of limiting the costs of this adaptation through, for example, letting it affect as few and as inexpensive parts as possible.
In most known designs of progressive cavity pumps with inner and outer rotors is required—unless the pumping medium is of a kind which may be allowed to penetrate into the bearings of the outer rotor or even function as an active component in hydrodynamic bearings—a large diameter on the dynamic seals of the outer rotor with consequent relatively large leakage, frictional moment and hydrostatic axial forces on the bearings of the outer rotor. A reason for the big seal diameter is that the seal normally surrounds the entire helical cavity with Z+1 thread starts and that this cross section cannot be reduced towards the seal if the inner rotor is to be installable from the same side as the seal and if the outer rotor is to be made in one piece. With this typical construction there will also be an unfavourable flow pattern as pumping medium is let in and out, because the medium meets the plane end surface of the outer rotor as an obstruction vertically to the direction of flow.
The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art.
The object is achieved through features which are specified in the description below and in the claims that follow.